In a stage control system using a laser interferometer, an interferometer which measures the position of a stage generally has one measurement axis per one degree of freedom of a driving shaft, as disclosed in, e.g., Japanese Patent Laid-Open No. 10-289943. However, to increase the driving stroke of the stage in this control system, a reflection mirror which reflects measurement light from the interferometer needs to have a length corresponding to that of the driving stroke. Consequently, the mass of the mirror to be mounted on the stage increases to decrease the natural frequency, and an increase in dynamic characteristics of the stage cannot be expected.
Japanese Patent Laid-Open No. 2001-23891 discloses an example wherein a plurality of laser interferometers are provided for one-axis position measurement to measure the position of a stage. Since the measurement optical axes of the plurality of interferometers strike a reflection mirror in this arrangement, the reflection mirror need not have a size corresponding to that of the movable stroke of the stage. The switching positions of the plurality of interferometers have a hysteresis with respect to the traveling direction, thereby avoiding chattering in switching operation in the vicinity of each interferometer switching position.
In Japanese Patent Laid-Open No. 2000-187338 as well, a plurality of laser interferometers are provided for one axis of the stage driving stroke of an exposure apparatus. Switching is performed between the interferometers in off-axis alignment measurement and in exposure, thereby reducing the weight of a reflection mirror for the interferometers.
To measure the position of the stage of an object to be measured (e.g., an exposure apparatus) by switching between two interferometers, generally, a stroke is set within which the two interferometers can simultaneously measure the position of the stage, and the measurement value of one interferometer as the switching source is preset to the measurement value of the other interferometer measuring the current position of the stage when the measurement positions of the two interferometers fall within the stroke. The same applies to a case wherein interferometer switching is performed twice or more. Assume that the measurement axis in use is switched from the measurement axis of one interferometer to that of another while a stage to be controlled is moving. In this case, an error of a certain magnitude proportional to the moving velocity occurs unless a period of time from when the measurement value of one interferometer is read to when the measurement value of another interferometer is preset to the read measurement value is 0. Variations in periods of time required for the presetting cause an error to have an indefinite magnitude, and the error magnitude of the current position held by each interferometer accumulates. Under the circumstances, to avoid these problems, various remedies are adopted. For example, the number of times of interferometer switching in processing a wafer by an exposure apparatus is reduced or interferometer switching is performed while a wafer stage is stopped during baseline measurement.
However, in the above-mentioned remedies, an interferometer switching position needs to be arranged at a specific position such as a baseline measurement position limited by the system. This imposes constraints on system design. For this reason, a control system capable of performing switching between interferometer measurement axes at arbitrary positions has been desired.
In a control system which has a position measuring device including, e.g., an interferometer, mode separation is performed to generate command position coordinates on the basis of the current position of the stage or an interferometer measurement value in order to set a stage target value (to be referred to as a target value in an abstract coordinate system hereinafter), which is used in the higher sequence to the value of a prototype serving as a reference of a scale and perpendicularity or to eliminate interference from other axes. A conversion calculation between an interferometer measurement value (RAW data of each laser interferometer counter in FIGS. 5 and 12) and a value in an abstract coordinate system will be referred to as “fine correction” hereinafter. A correction parameter set is provided for each stage driving axis and is used to adjust for each device a deviation from a true value, which is generated due to the assembly tolerance and processing error of the control system, by self-measurement or external measurement.
To eliminate any steady-state deviation in scan driving at a uniform velocity, a wafer stage used for a scan exposure apparatus tends to be adjusted such that high feedback gain is obtained in a low-frequency region. Assume that a deviation from the target value of the wafer stage changes stepwise in the control system like this. In this case, even if the amount of deviation is as small as about several μm, a current command value issued from the stage to a motor may saturate. As a result, various troubles may be induced. More specifically, the control system may become unstable or the protective circuit of the motor may operate. Assume that at the instant when the laser interferometer in charge of measuring the current position of the wafer stage is switched from one laser interferometer to another, an interferometer measurement value to be handed over is discontinuous or a current interferometer has a different correction parameter set for correcting an interferometer measurement value from that of a succeeding interferometer. In this case, even if the target position issued from the higher sequence to the wafer stage is constant, a deviation amount from the target value may change stepwise. This variation in position information will be described below by taking magnification correction of an interferometer measurement value as an example. Since the magnification of an interferometer has a high absolute precision, there is generally not much need for correction for each interferometer. For the sake of descriptive simplicity, magnification correction will be described as the simplest example.
Let L1 be a measurement value of a current interferometer 1, X1 be the current position of a stage, L2 be a measurement value of a succeeding interferometer 2, X2 be the current position of the stage, ML1 be a magnification correction amount from the reference scale of the interferometer 1, ML2 be a magnification correction amount from the reference scale of the interferometer 2, and X0 be an origin offset amount in the X-axis direction. When switching from the interferometer 1 to the interferometer 2 is to be performed, the measurement value of the interferometer 2 is preset to the measurement value of the interferometer 1 while the stage is stopped. In this case, a variation (ΔX) in current position in an abstract coordinate system is represented by:X1=L1×(ML1+b)+X0  (1)X2=L2×(ML2+b)+X0  (2)
Since L2 is preset to L1, L1=L2 holds.ΔX=X2−X1=L1(ML2−ML1)  (3)
The variation ΔX represented by equation (3) changes depending on the position of the stage. For this reason, current saturation may occur at some interferometer switching positions with respect to interferometer measurement values.
As can be seen from the above description, in a stage having a plurality of interferometers per axis, the interferometers need to be precisely arranged such that each interferometer has the same measurement conditions for the exit angle and dead path length (optical length as the position measurement target) of measurement light of the interferometer illuminating an interferometer reflection mirror of the stage. However, it is difficult to precisely arrange these interferometers so as to attain the required precision.